The invention relates to sodium-resistant joining glasses, which can be used to produce joints with ceramics, for example alumina (also referred to as Al2O3, alumina ceramic or Al2O3 ceramic), and/or with metals and/or combinations of both. The invention as well relates to the application of the sodium-resistant joining glasses. Sodium-resistant joining glasses are joining materials which can withstand in particular liquid sodium and/or sodium vapour and in this way are suitable for the production of joints which are exposed, for example, to liquid sodium and/or sodium vapour and/or other aggressive sodium compounds and/or sodium-containing media.
Sodium-resistant joining glasses of this type are of interest, for example, for the production of energy storage and energy generation units, in which liquid sodium and/or sodium compounds are used as the electrolyte or cooling medium. Examples of such energy generation units are electrochemical cells such as batteries as well as nuclear reactors, especially within the class of Fast Breeder Reactors, Fast Neutron Reactors, Sodium-Cooled Fast Reactors and/or Liquid Metal Fast Breeder Reactors. The joining glasses useable in those reactors could be advantageously applied in technical components within and/or connected to those reactors and being in contact with said sodium and/or sodium compounds. Examples of such technical components are especially feedthrough-devices, which are used to supply sensors and/or actuators and/or electric motors e.g. in electric pumps with electrical power and/or steering signals. Another application area of the sodium-resistant joining glasses being subject to this invention are installations for the disposal of toxic materials, in which sodium compounds and/or sodium vapour and/or liquid sodium might be produced within the process.
Especially the electrochemical storage and energy generation technology has gained considerable interest in recent years. It can be employed in this respect in the field of electromobility, for local power supply, as an emergency power system and, primarily on account of the increased shares of renewable energies, for stabilizing the network system.
Various battery technologies are under discussion here, with the Li-ion batteries (LIB) being those which are discussed to the greatest extent. A further class of batteries is represented by the high-temperature sodium batteries (Sodium Beta Battery, SBB). The advantages thereof over the LIB are the higher energy density and high energy efficiency. The SBB uses liquid sodium as the negative electrode at elevated temperatures, usually of more than 250° C. A distinction is made in general terms between two variants: one is the sodium-sulphur battery (Na/S), which uses sulphur as the positive electrode. The other is the sodium-metal chloride battery, also referred to as ZEBRA® battery, which uses metal chlorides such as nickel or iron chloride as the positive electrode and sodium tetrachloroaluminate (NaAlCl4) as the liquid electrolyte. Both types have the common feature that they use a sodium ion-conducting membrane consisting of β- or β″-Al2O3 and a housing part consisting of α-Al2O3, and the latter can if appropriate additionally be connected to a metallic cover.
The generic term “alumina” used in the present description, or synonymously alumina ceramic or synonymously Al2O3 or likewise synonymously Al2O3 ceramic, includes in particular the embodiments α- and/or β- and/or β″-alumina. The use of the term “alumina” also does not signify any limitation to the degree of purity and therefore the content of Al2O3 in the Al2O3 ceramic and/or the component in question.
The joint between the components made of ceramic, in particular alumina, or a further metal component represents a critical component in an electrochemical cell, since it determines the service life. If leakages occur in this region, the liquid sodium can come into contact with the atmosphere and begin to burn. The object of the joining glass as the joining material here is to achieve a hermetically tight join which lasts for the entire service life of the battery. This can be achieved in particular by a good adaptation of the coefficient of thermal expansion of all the materials involved, which makes the joint tolerant to the operating states, and also a very good chemical resistance of the glass to all active components, without impairing the function thereof.
A distinction is made between two fundamental types of joining glasses for batteries: silicate-based glasses and borate-based glasses. Borate-based glasses have the advantage that they usually have a very good resistance to the molten sodium, but they have poorer properties in terms of their chemical resistance to the metal chloride. In addition, the aluminoborates which are frequently used often only have a low stability against crystallization, and this limits them from the viewpoint of process control. A special form is specified, for example, in U.S. Pat. No. 8,334,053 B2, which describes separate glasses depending on corrosion resistance on anode and cathode of an SBB. According to said document, a glass with a high silicon content comprising more than 40% by weight SiO2 and less than 25% by weight B2O3 is used on the metal salt side, and a borate glass with a very low silicon content of less than 20% by weight SiO2 and more than 35% by weight B2O3 is used on the sodium side.
GB 2207545 A describes the use of a borosilicate glass 8245 from Schott AG as a joining glass for a Na/S battery. This glass has a very good chemical stability with respect to the media of the Na/S battery, but can only be hermetically joined durably to a limited extent with alumina owing to the low coefficient of linear thermal expansion α20-300° C. of 5.2·10−6 K−1.
U.S. Pat. No. 4,268,313 A describes a borosilicate glass for use in an Na/S battery. However, this glass comprises in total at least 6% by weight of the alkaline earth metal oxides CaO, SrO and BaO. These components are beneficial to the glass formation and can improve the flow behaviour, but can reduce the performance of the active components by ion transfer with the electrolyte, in particular of an SBB.
A joining glass containing at most 25% by weight B2O3 for an energy storage device is described in U.S. Pat. No. 8,034,457 B2. The limitation of the content of 82O3 to said upper limit is explained by the fact that the joining glass would otherwise be subjected to excessive attack by adsorbed water.
U.S. Pat. No. 8,043,986 B2 includes a joining glass for an SBB comprising at least 0.1 to 10% by weight ZrO2. Zirconium oxide is used in this document for improving the chemical resistance. However, it also leads to a greater tendency toward phase separation and crystallization and also, on account of the high raw material costs, to a reduction in the efficiency of the glassmaking process.